The results are recorded of an investigation of the geographical distribution of West Nile virus in Egypt and the southern Sudan, the epidemiology of human infection, and the general ecology of the virus in the Nile Delta, as revealed by virus isolations, surveys for specific antibodies and study of the biological environment.
According to serological surveys the virus is widely disseminated along the Egyptian Nile and in the southern Sudan; 61 per cent of 1168 human bloods collected in Egypt and 40 per cent of 350 collected in the Sudan neutralized WN virus.
In an endemic area in the Nile Delta where the intensive studies were conducted, West Nile infection is essentially a childhood disease with yearly peaks of transmission during mid-summer. No reliable information on the symptomatology of the infection in children was obtained, but the impression was gained that it is a self-limited, non-fatal febrile disease rarely associated with definite manifestations of encephalitis.
The virus was repeatedly isolated from the blood of febrile children (23) and from mosquitoes, Culex univittatus (12) and Culex antennatus (5), twice from pigeons (Columba livia) and once from the hooded crow (Corvus corone sardonius). The fact that C. univittatus was more frequently found to be infected than any other species of mosquito may be due to its relative predilection for avian blood. None of 530 pools of other arthropods, including ticks, lice, fleas, mites and flies, yielded virus.
On both experimental and ecological grounds it is concluded that the main cycle of the virus is probably through mosquitoes and birds in which man may become involved. Serological evidence points to infections of domestic quadrupeds, but these are probably only tangential or dead-end infections.
It is likely that non-migratory birds are responsible for the local dissemination of the virus but, as it appears to be endemic in parts of the Nile Delta, its transport over long distances by migratory birds was not investigated. There is evidence for believing that the virus over-winters through a process of retarded transmission by the mosquitoes that remain active throughout the colder months, although the possible role of ticks as a reservoir of the virus should be further investigated. Non-endemicity in the northern rim of the Delta may be accounted for by the less dense human and bird population there and the scarcity of what appears to be the most important vector, C. univittatus.
Present address: Yale University Medical School, 333 Cedar St., New Haven, Connecticut.